vl400 and 2x256Mb

I was lead to believe that the vectra vl400 could support 512MB of RAM in the form of two 256MB sticks. So I got two 256MB SDRAM PC133 32Mx64 Low Density sticks. When I set one in, it boots fine, and assumes 256MB memory. But if I set them both, the e-Buzzer gives a memory error code (long ringing beep, then 3 beeps).